JP2813790B2 - Method for cleaning and drying compound semiconductor single crystal substrate - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a technique for cleaning and drying a semiconductor substrate, and more particularly to a technique suitable for use in cleaning and drying performed after mirror polishing a GaAs single crystal substrate. It is an effective technique for preventing heavy metal contamination.

[0002]

2. Description of the Related Art Conventionally, a GaAs single crystal substrate (wafer) that has been subjected to mirror polishing is melted to remove wax residues used for fixing the substrate during polishing and dust adhering to the substrate surface. After selecting a suitable organic solvent from among the organic solvents such as trichlene, acetone, methanol, etc. in consideration of the properties, or washing the substrate with a solvent combining them, the organic solvent and water-soluble contamination with ultrapure water After washing off the substance, the substrate surface is dried by spin drying or blow drying by blowing nitrogen gas. In the spin drying, the single crystal substrate is rotated at a high speed using a spinner or the like to drain the surface.

[0003]

However, in the above-mentioned conventional method, mottled stains called so-called uneven drying may occur on the substrate surface after drying, and the quality of the substrate surface may be degraded. Such drying unevenness is an aggregate of fine particles of submicron to about 10 micron, and is firmly attached to the substrate surface. Therefore, it is necessary to remove the unevenness even after spraying with nitrogen gas or washing again with ultrapure water. There was a disadvantage that it could not be done.

Further, the conventional method has a disadvantage that the substrate is oxidized in the course of cleaning with ultrapure water after polishing, and an oxide film having a relatively large thickness and a large in-plane variation is formed on the surface. Was. For this reason, conventionally, etching was performed using a sulfuric acid-based etchant immediately before the epitaxial growth. However, this etching method often causes variations in the amount of etching,
Since the flatness of the substrate surface is impaired and an oxide film is formed again after the etching, the surface oxide film of the substrate cannot be completely removed. In addition, it has been found that the thicker the surface oxide film, the higher the surface concentration of heavy metal as a contaminant.

[0005] The residual oxide film can be removed to some extent by heat-treating (thermal etching) the substrate immediately before the epitaxial growth. However, in such a heat treatment, a part of the oxide film is sometimes removed. It has been found that this is not preferable because it remains or causes a surface defect of the substrate. In addition, even if the surface oxide film is thermally etched, even the heavy metal cannot be removed, so if there is a lot of heavy metal contamination, the heavy metal remains in a sheet form at the interface with the substrate when the epitaxial layer is grown on the substrate surface,
There is a drawback that when an electronic device is created, it acts as a trap to slow down the operation speed.

On the other hand, the applicant of the present invention has previously proposed a steam drying method in which water on the substrate surface is replaced with alcohol using isopropyl alcohol or the like to prevent the drying unevenness described above (Japanese Patent Application No. 3-150893). issue). However, the above-mentioned steam drying method is effective for eliminating drying unevenness, but since water and impurities are accumulated in a solvent such as isopropyl alcohol, it is necessary to exchange the solvent at a considerable frequency, which increases the cost and increases the cost. , Fe, Cu, Ni, and other heavy metals may spread over the entire surface of the substrate.

SUMMARY OF THE INVENTION The present invention has been made in view of the above-mentioned problems, and an object of the present invention is to provide a simple substrate cleaning method using only spin drying or blow drying without using a steam drying method and having no drying unevenness. It is to provide a method. Another object of the present invention is to provide a substrate cleaning method in which the thickness of an oxide film on a substrate surface is small and uniform. Still another object of the present invention is to provide a method of cleaning a substrate with less heavy metal contamination on the substrate surface.

[0008]

Means for Solving the Problems The inventors of the present invention have found that, during the cleaning process using ultrapure water, the substrate is oxidized to form a highly scattered surface oxide film because the dissolved oxygen concentration in the ultrapure water is low. Considering that it may be related, the condition of ultrapure water used for cleaning after the etching was examined. FIG. 1 shows the relationship between the concentration of dissolved oxygen in ultrapure water and the thickness of the oxide film remaining on the substrate surface obtained by the experiment. As shown in the figure, by setting the concentration of dissolved oxygen in ultrapure water to 1.0 ppm or less, the thickness of the oxide film can be reduced to 2 nm or less. It can be seen that the thickness can be reduced to 0.5 nm or less. However, considering that there is oxygen dissolved in ultrapure water during washing, it is preferable to use ultrapure water having a dissolved oxygen concentration of 0.5 ppm or less.

Further, the in-plane distribution of the oxide film thickness on the substrate surface was examined using an ellipsometer, and the in-plane distribution of heavy metals on the substrate surface was examined by total reflection X-ray fluorescence analysis. It was found that the in-plane distribution and the in-plane distribution of heavy metal corresponded. From this, the heavy metal is localized in the surface oxide film, and if the oxide film can be removed during cleaning, the concentration of the heavy metal can be reduced. A cleaning experiment was performed with ultrapure water having a reduced water content. As a result, it has been found that the use of ultrapure water having a dissolved oxygen concentration of 0.5 ppm or less, preferably 0.1 ppm or less can reduce the heavy metal concentration to 1 × 10 ¹⁰ atoms / cm ² or less.

On the other hand, as a result of examining the oxide film on the substrate surface by XPS (X-ray photoelectron spectroscopy), the unevenness of drying corresponds to the portion where the ratio of As contained in the oxide film is high. Probably, dissolved oxygen in water droplets remaining on the substrate surface reacts with GaAs to produce As ₂ O ₃ , and this As ₂ O 3
It is considered that No. ₃ was concentrated and precipitated during drying. Incidentally, in the above-mentioned Japanese Patent Application No. 3-150893, dry unevenness is also assumed to be As ₂ O ₃ fine particles.

Therefore, if the above-described cleaning with ultrapure water having a reduced dissolved oxygen concentration is performed, the dissolved oxygen in the water droplets becomes Ga
The above experimental results were reexamined on the assumption that it was possible to prevent As ₂ O ₃ from reacting with As and remaining as drying unevenness.
As a result, when the dissolved oxygen concentration of the ultrapure water was higher than 0.5 ppm, drying unevenness occurred, and the dissolved oxygen concentration was reduced to 0.1 ppm.
It has been found that when 5 ppm to 0.1 ppm, uneven drying does not occur, and when the dissolved oxygen concentration is less than 0.1 ppm, uneven drying may occur depending on the case. When the concentration of dissolved oxygen is less than 0.1 ppm, uneven drying occurs because such ultrapure water has too good wettability with the substrate. This is considered to be because drying takes a long time. Therefore, even when the dissolved oxygen concentration is less than 0.1 ppm, the cleaning time with ultrapure water is lengthened to form a surface oxide film more uniformly, or a method of spin drying, for example, rotating a single crystal during spin drying. It is possible to eliminate uneven drying by improving the drainage by devising conditions such as the speed of rotation and the time for rotation. The ultrapure water is obtained by removing impurities such as metal ions dissolved in water, and the quality thereof is controlled so that the electric resistivity is at least 5 MΩ · cm, usually 10 MΩ · cm or more. . Ultrapure water having a low dissolved oxygen concentration can be obtained by, for example, blowing nitrogen gas into the ultrapure water containing no impurities. Note that a high-purity nitrogen gas is used so as to meet the above purpose.

The present invention has been made on the basis of the above-described findings. In cleaning a mirror-polished compound semiconductor single crystal substrate washed with an organic solvent with ultrapure water, the dissolved oxygen concentration is 0.5 ppm or less. Washing is performed using ultrapure water and spin drying or blow drying is performed. Furthermore,
As another embodiment of the above invention, the following invention is provided.
That is, when the compound semiconductor single crystal substrate that has been mirror-polished and washed with an organic solvent is washed with ultrapure water, first, washing is performed using ultrapure water having a dissolved oxygen concentration of 0.1 ppm or less, and then dissolved oxygen is used. The concentration is 0.1 ppm or more.
After washing using ultrapure water of 5 ppm or less, spin drying or blow drying is performed. In the invention, first, cleaning is performed using ultrapure water having a dissolved oxygen concentration of 0.1 ppm or less, thereby mainly removing heavy metals remaining on the substrate surface.
By performing the cleaning using ultrapure water of not less than pm and not more than 0.5 ppm, the oxide film remaining on the surface after the completion of the cleaning with the ultrapure water is reduced, and the occurrence of uneven drying is suppressed.

[0013]

According to the above-described means, it is possible to easily carry out substrate cleaning without uneven drying by only spin drying or blow drying without using a steam drying method. According to the above-mentioned means, the thickness of the surface oxide film on the substrate is very thin and uniform, 2 nm or less, and the surface concentration of the heavy metal after cleaning is 1 × 10 ¹⁰ atoms / cm ² or less. A substrate with less heavy metal contamination can be obtained.

[0014]

【Example】

(Example 1) A GaAs single crystal grown by the LEC method was cut into a thin plate having a thickness of 1 mm along the (100) plane, and then subjected to rough polishing and mirror polishing. Then, in order to remove wax residues used for fixing the substrate and dust adhering to the substrate surface, organic cleaning with trichlene, acetone, and methanol was performed. Next, a final cleaning with ultrapure water having a dissolved oxygen concentration of 0.5 ppm was performed for 5 minutes to wash off the organic solvent, and then the substrate surface was dried by a spin dryer.

[0015] 100 GaAs single crystal substrates were washed and dried by the above method. After drying, the condition of the surface of the substrate was visually inspected. As a result, there was no substrate having uneven drying on the surface. The number of fine particles adhering to the substrate surface was measured by a laser beam irradiation type particle counter. As a result, the average number of fine particles having a diameter of 0.2 μm or more was 1 per 1 cm ² .

(Example 2) 100 GaAs single crystal substrates which had been subjected to organic cleaning in the same process as above were washed with ultrapure water having a dissolved oxygen concentration of 0.05 ppm, which is the lower limit of measurement, and then washed. Continue to increase dissolved oxygen concentration by 0.3pp
After washing with ultrapure water having a diameter of m, spin drying was performed. The total of the first and second cleaning times was 5 minutes, the same as in Example 1. After drying, the condition of the substrate surface was visually inspected.
None of the substrates had uneven drying on the surface of the substrate. Further, the substrate on which uneven drying occurred was only slightly present in the outer garden of the substrate. The number of fine particles adhering to the substrate surface was measured by a laser beam irradiation type particle counter. As a result, the number of fine particles having a diameter of 0.2 μm or more was 1 per 1 cm ² as in Example 1.

(Example 3) The 100 GaAs single crystal substrates which had been subjected to the organic cleaning in the same process as described above were subjected to final cleaning with ultrapure water having a dissolved oxygen concentration of 0.05 ppm, which is the lower limit of measurement, for 5 minutes. Run and spin dry. After the drying, the surface of the substrate was visually inspected, and as a result, no drying unevenness occurred on the surface of the 80 substrates. In addition, even in the case of the 20 substrates on which uneven drying occurred, only slight unevenness was present on the outer edge of the substrate. Further, the number of fine particles adhering to the substrate surface was measured by a laser beam irradiation type particle counter on a substrate without drying unevenness. As a result, the number of fine particles having a diameter of 0.2 μm or more per 1 cm ^{2 on} average was the same as in Example 1. There was one.

COMPARATIVE EXAMPLE 100 GaAs single-crystal substrates that had been subjected to organic cleaning in the same steps as described above were subjected to final cleaning with ultrapure water having a dissolved oxygen concentration of 8 ppm for 5 minutes and spin-dried. After the drying, the condition of the substrate surface was visually inspected. As a result, drying unevenness occurred on the surface of all 100 substrates. Further, as a result of measuring the number of fine particles adhering to the substrate surface using a laser beam irradiation type particle counter, the number of fine particles having a diameter of 0.2 μm or more was larger than in Examples 1, 2, and 3.

Next, with respect to the washed and dried substrates of Examples 1, 2 and 3 and Comparative Example, the respective concentrations of heavy metals on the surface were measured by a total reflection X-ray fluorescence analyzer. Table 1 shows the average of the measured values of the surface concentrations of Fe, Ni, Cu, and Zn, which are representative heavy metals. In the substrates of Example 1, Example 2 and Example 3, Fe, N
The concentration of i and Cu is 1 × 10 ¹⁰ atoms / cm ^{2 which} is the lower limit of measurement.
It was below. Although it cannot be distinguished in Table 1, in Example 2, since the cleaning was performed using ultrapure water having a lower dissolved oxygen concentration at first, the heavy metal concentration is expected to be lower than that in Example 1.

[0020]

[Table 1]

Further, the thicknesses of the surface oxide films of the cleaned and dried substrates of Examples 1, 2 and 3 and Comparative Example were measured. In Examples 1, 2 and 3, ２2 nm, and 5-10 nm in the comparative example. In the above embodiment, a method for cleaning a GaAs single crystal substrate has been described. However, the present invention is not limited to this, and can be used for cleaning an InP single crystal substrate and other compound semiconductor single crystal substrates. In the above embodiment, the GaAs single crystal substrate is dried by spin drying, but blow drying by blowing an inert gas can be used.

[0022]

As described above, according to the present invention, when a compound semiconductor single crystal substrate which has been mirror-polished and washed with an organic solvent is washed with ultrapure water, the dissolved oxygen concentration is 0.5%.
Since cleaning is performed using ultrapure water of less than ppm and spin drying or blow drying is performed, substrate cleaning without drying unevenness can be easily performed only by spin drying or blow drying without using a steam drying method. This has the effect of avoiding an increase in cost. In particular, a semiconductor substrate is desired to have a large diameter, and a large diameter substrate requires a large amount of a solvent for drying. Therefore, the present invention will be more and more useful as compared with a steam drying method.
The substrate to which the present invention is applied has a surface oxide film having a thickness of 2 mm.
Since it is extremely thin and uniform, ie, nm or less, it is possible to perform epitaxial growth without performing immediately preceding etching, so that a compound semiconductor single crystal substrate suitable for epitaxial growth can be obtained. Further, the substrate to which the method of the present invention is applied has a heavy metal surface concentration of 1 × 1 after cleaning.
0 ¹⁰ atoms / cm ² or less, and there is little contamination of heavy metals, so that when an epitaxial layer is grown on the substrate surface, no heavy metals remain at the interface with the substrate, improving the characteristics of electronic devices using this as a substrate. Can be done.

[Brief description of the drawings]

FIG. 1 is a graph showing the relationship between the concentration of dissolved oxygen in ultrapure water used for cleaning and the thickness of an oxide film remaining on a substrate surface.

Continuation of the front page (56) References JP-A-4-40270 (JP, A) JP-A-3-157927 (JP, A) JP-A-5-291233 (JP, A) JP-A-5-291231 (JP) , A) JP-A-4-350934 (JP, A) (58) Fields investigated (Int. Cl. ⁶ , DB name) H01L 21/304

Claims (2)

(57) [Claims] When a compound semiconductor single crystal substrate that has been mirror-polished and washed with an organic solvent is washed with ultrapure water, washing is performed using ultrapure water having a dissolved oxygen concentration of 0.5 ppm or less, and spin drying or A method for cleaning and drying a compound semiconductor single crystal substrate, comprising performing blow drying. 2. When a compound semiconductor single crystal substrate which has been mirror-polished and washed with an organic solvent is washed with ultrapure water, the compound semiconductor single crystal substrate is first washed with ultrapure water having a dissolved oxygen concentration of 0.1 ppm or less, and then washed. Washing and drying using ultrapure water having a dissolved oxygen concentration of 0.1 ppm or more and 0.5 ppm or less, followed by spin drying or blow drying.